1. Field of the Invention
This invention relates to a fluid reservoir for a master cylinder for use in brake and clutch systems of a vehicle and particularly to such a reservoir provided with a diaphragm.
2. Prior Art
One known master cylinder reservoir comprises a cylindrical body having an open upper end, the body having fluid feed ports at its lower portion, a cap removably secured to the open end of the reservoir body, for example, through a threaded connecton, and a diaphragm received in the cap and having a peripheral portion held between the upper surface of the open end of the reservoir body and a mounting portion of the cap disposed in opposed relation to the upper surface of the open end. The diaphragm is made of an elastic material such as rubber so that it can be deformed when subjected to a pressure differential created thereacross. When assembling the master cylinder reservoir, a required amount of fluid is introduced into the reservoir body. Then, the cap is rotated relative to the reservoir body to make the threaded connection therebetween with the peripheral portion of the diaphragm being interposed between the upper surface of the open end of the reservoir body and the mounting portion of the cap, so that the reservoir body is sealed by the diaphragm. As the cap is rotated relative to the reservoir body, the peripheral portion of the diaphragm is subjected to a distortion or deformation due mainly to a friction between the upper surface thereof and the mounting portion of the cap and a friction between the lower surface thereof and the upper surface of the open end of the reservoir body. This distortion affects the sealing of the reservoir body by the diaphragm. To overcome this problem, it has been proposed to mount a slide ring, made of a material having a low coefficient of friction such as a plastics material, between the upper surface of the peripheral portion and the mounting portion of the cap. The reason why the slide ring is mounted only on the upper surface of the peripheral portion of the diaphragm is that the lower surface of the peripheral portion must be held in sealing engagement with the upper surface of the open end of the reservoir body while the upper surface of the peripheral portion does not need to be held in sealing engagement with the mounting portion of the cap. For example, the master cylinder reservoir provided with such a slide ring is shown in U.S. Pat. No. 4,103,492. However, unless the slide ring is positioned in place when attaching the cap to the reservoir body, the slide ring does not achieve the intended purpose. In addition, the assembly of the master cylinder reservoir become rather cumbersome and requires more time and labor because the component parts are increased by the use of the slide ring.
To overcome this difficult, it has also proposed to provide projections on the mounting portion of the cap which projections are snugly fitted respectively in notches formed in the peripheral portion of the diaphragm. With this construction, the diaphragm is attached to the cap against rotation, and the projections are of such a height that the lower surfaces of the projections are brought into engagement with the upper surface of the open end of the reservoir body, thereby preventing an undue distortion or deformation of the peripheral portion of the diaphragm. Such a master cylinder reservoir is disclosed in Japanese Utility Model Application No. 57-138,585 filed on Sept. 13, 1982 and laid open to public inspection on Mar. 21, 1984. However, the compressed peripheral portion interposed between the cap and the reservoir body is still forced radially inwardly when the master cylinder reservoir is assembled, so that the diaphragm is not positioned in place between the cap and the reservoir body. This problem will not occur if the cap is not tighened hard relative to the reservoir body. However, the master cylinder mounted on a vehicle is subjected to substantial vibration, and therefore in practical use, the cap must be tightened hard enough to ensure the sealing of the reservoir body by the diaphragm. Since the cap and the reservoir body are made of plastics material, they are subjected to a deformation when tightening the cap relative to reservoir body. As a result, the peripheral portion of the diaphragm is subjected to an undue deformation despite the provision of the projections.